1. Technical Field
The present invention relates to a refrigerant compressor, and more particularly, to a slant plate type compressor, such as a wobble plate type compressor, with a variable displacement mechanism suitable for use in an automotive air conditioning system.
2. Description of the Prior Art
It has been recognized that it is desirable to provide a slant plate type piston compressor with a displacement or capacity adjusting mechanism to control the compression ratio in response to demand. As disclosed in U.S. Pat. No. 4,428,718, the compression ratio may be controlled by changing the slant angle of the sloping surface of a slant plate in response to the operation of a valve control mechanism. The slant angle of the slant plate is adjusted to maintain a constant suction pressure in response to a change in the heat load of the evaporator of an external circuit including the compressor or a change in rotation speed of the compressor.
In an air conditioning system, a pipe member connects the outlet of an evaporator to the suction chamber of the compressor. Accordingly, a pressure loss occurs between the suction chamber and the outlet of the evaporator which is directly proportional to the "suction flow rate" therebetween as shown in FIG. 8. As a result, when the capacity of the compressor is adjusted to maintain a constant suction chamber pressure in response to appropriate changes in the heat load of the evaporator or the rotation speed of the compressor, the pressure at the evaporator outlet increases. This increase in the evaporator outlet pressure results in an undesirable decrease in the heat exchanging ability of the evaporator.
The above mentioned U.S. Pat. No. 4,428,718 discloses a valve control mechanism, to eliminate this problem. The valve control mechanism, which is responsive to both suction and discharge pressures, provides controlled communication of both suction and discharge fluid with the compressor crank chamber and thereby controls compressor displacement. The compressor control point for displacement change is shifted to maintain a nearly constant pressure at the evaporator outlet portion by means of this compressor displacement control. The valve control mechanism makes use of the fact that the discharge pressure of the compressor is roughly directly proportional to the suction flow rate.
However, in the above-mentioned valve control mechanism, a single movable valve member, formed of a number of parts, is used to control the flow of fluid both between the discharge chamber and the crankcase chamber, and between the crankcase chamber and the suction chamber. Thus, extreme precision is required in the formation of each part and in the assembly of the large number of parts into the control mechanism in order to attempt to ensure that the valve control mechanism operates properly. Furthermore, when the heat load of the evaporator or the rotation speed of the compressor is changed quickly, the discharge chamber pressure increases and an excessive amount of discharge gas flows into the crank chamber from the discharge chamber through a communication passage of the valve control mechanism, due to a lag time to between the operation of the valve control mechanism in response to the external circuit including the compressor. As a result of the excessive amount of discharge gas flow, a decrease in compression efficiency of the compressor, and a decline of durability of the compressor internal parts occurs.
To overcome the above-mentioned disadvantage, Japanese Patent Application Publication No. 1-142276 proposes a slant plate type compressor with a variable displacement mechanism which is developed to take advantage of the relationship between discharge pressure and suction flow rate. That is, the valve control mechanism of this Japanese '276 publication is designed to have a simple physical structure and to operate in a direct manner on a valve controlling element in response to discharge pressure changes, thereby resolving the complexity, excessive discharge flow and slow response time problems of the prior art.
However, in both the U.S. '718 Patent and Japanese '276 publication, the valve control mechanism maintains pressure in the evaporator outlet at a predetermined desired value by means of compensating for the pressure loss occurring between the evaporator outlet and the compressor suction chamber, in direct response to the pressure in the compressor discharge chamber, as shown in FIG. 7. That is, the pressure at the evaporator outlet is maintained constant as the discharge pressure increases, and as a result, the pressure in the suction chamber is decreased in order to compensate for the pressure loss between the evaporator outlet and the suction chamber. Thus, the pressure of the evaporator is maintained constant in dependence only on the magnitude of the discharge pressure, and other factors such as the pressure in the suction chamber and the external operating conditions of the air conditioning circuit are not taken into account. Furthermore, when, the displacement of the compressor is controlled in response to characteristics of the automotive air conditioning system, such as, the temperature of passenger compartment air or the temperature of air leaving the evaporator in addition to the change in the heat load of the evaporator or the change in rotation speed of the compressor, which is desired in order to more effectively operate the automotive air conditioning system, the pressure loss in the suction chamber must be compensated for by some further mechanism in order to avoid a loss in efficiency. Therefore, the above-mentioned technique of the prior art, in which the pressure loss in the suction chamber is not compensated for is not suited to elaborate operation of the automotive air conditioning system.